1. Field of the Invention
This invention relates to data logging. More particularly, this invention relates to data logging systems that store the logged data on a storage medium using data compression by logging a sequence of data values and the times for which they subsist.
2. Description of the Prior Art
It is known from U.S. Pat. No. 4,616,320 to provide a seismic activity logging system in which seismic data is recorded in compressed form into a long term memory comprising an integrated circuit memory. The seismic data is continuously recorded into a buffer memory. If the seismic signals exceed a preset level then the contents of the buffer and any further signals are stored in the long term memory until the seismic signal falls below a preset level for a preset period. The data is stored in compressed form by recording a signal value and a rate of change value starting from the signal value together with a time for which the actual signal remains within a preset tolerance of the value that would be predicted by extrapolating from the signal value with the rate of change value. When the tolerance is exceeded, a new point is recorded. This is analogous to runlength coding.
A problem within the data logging field in general and the above described system is that as the degree of compression increases there is an increase in the loss of detail within the data recorded.